Particulates or particulate matter discharged from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbon and contains a trace of sulfate (misty sulfuric acid fraction). In order to suppress such kind of particulates from being discharged into atmosphere, it has been envisaged as shown in FIG. 1 that a particulate filter 4 is incorporated in an exhaust pipe 3 through which exhaust gas 2 from a diesel engine 1 flows.
As detailedly shown in FIG. 2, the particulate filter 4 comprises a filter body 7 in the form of a porous honeycomb made of ceramics such as cordierite. The filter body 7 has passages 5 in the form of grid-like compartments with inlets alternately plugged by plugs 8, the passages 5 with the unplugged inlets being plugged at their outlets by the plugs 8. Only the exhaust gas 2 passing through porous thin walls 6, which compartmentalize the passages 5, is discharged downstream and the particulates are captured at inner surfaces of the walls 6.
The particulates in the exhaust gas 2 are thus captured by and accumulated on the inner surfaces of the walls 6 and spontaneously ignite to be burned off upon operational shifting to a region of operation with increased exhaust temperature. However, when an operation or driving with temperature at or above a predetermined temperature requisite tends not to continue for a long time, for example, in a vehicle such as a shuttle bus running mainly on congested city roads, there may be a fear that an accumulated particulate amount exceeds a treated particulate amount, disadvantageously resulting in clogging of the particulate filter 4.
Thus, provision of an electric heater has been investigated so as to satisfactorily burn off particulates even in a region of operation with lower exhaust temperature. Positive heating by this kind of electric heater will make it possible to satisfactorily burn off particles even in the region of operation with lower exhaust temperature.
As to heat regenerative particulate filters with electric heater, for example, the following Reference 1 has been proposed by the applicant of the present invention.
Reference 1: JP 62-255512A